Conventionally, in a semiconductor manufacturing process, a liquid crystal manufacturing process or the like, various gases are used according to the process. For instance, in a dry-etching step, a thin-film-forming step or the like, gases of PFC (perfluoro compound) that is a compound containing fluorine such as CF4, NF3, C2F6, C3F8, SF6, CHF3 and COF2, are used as a reactive gas, and an exhaust gas including them is produced.
An exhaust gas of these gases including the PFC gases is treated with various methods, because the gases have a high global warming potential and it is not preferable to discharge the exhaust gas outside in an untreated state. Such a treatment method includes a process of decomposing the PFC gas and removing its harmful effect by abatement treatment such as of a combustion type, a catalytic type, an adsorption type or a plasma decomposition type. However, in recent years, it is required to recover and reuse the PFC gas exhausted from a manufacturing process, from the viewpoint of environmental protection and energy savings.
When a PFC gas is exhausted from a manufacturing process, in general, the PFC gas is diluted with a large amount of nitrogen gas or the like so as to protect an exhaust line, a vacuum pump or the like, and is exhausted. At this time, the PFC gas is diluted to a concentration as low as several percent, though this depends on conditions. For this reason, when the PFC gas is subjected to harmful substance removal treatment, the PFC concentration process of removing nitrogen or the like from the exhaust gas is effective in order to remove nitrogen or the like that is unnecessary for the harmful substance removal treatment and increase treatment efficiency.
There is a method of separating a PFC gas from an exhaust gas by using a chromatographic separation technique that uses a chromatographic column containing various types of packing materials and reusing the separated PFC gas, as a method of concentrating and recovering the PFC gas. For instance, Japanese Patent Laid-Open Publication No. 2002-273144 proposes a gas separation apparatus which is directed at separating the specified gas from a gas to be treated containing the specified gas comprising a plurality of ingredients, and has a separation unit for chromatographically separating the specified gas from the gas to be treated by using a column packed with activated carbon.
In addition, other methods of concentrating and recovering the PFC gas include a membrane treatment method of separating the PFC gas from nitrogen or the like through a membrane, and a cryogenic distillation method of separating the PFC gas from nitrogen or the like by utilizing a difference between the boiling points of the PFC gas and nitrogen or the like.
For instance, technology for separating and recovering CF4 from nitrogen gas containing 0.05% CF4 by using a membrane treatment apparatus which employs polysulfonic polymer membranes stacked into three stages is described in “Development of Technology of PFC Recovery/Reuse, (2) Development for Practical Utilization of Technology”, Uemura Takashi, P 7, 9, [online], March, 2002, Semiconductor Leading Edge Technologies, Inc., PFC Final Debrief Session, [searched on Apr. 14, 2005], <URL:http://www.selete.co.jp/SeleteHPJ1/Data/200204/0204c05.pdf>.